A serial bus system is known for networking “intelligent” input/output units as well as sensors and actuators within an installation or machine. This serial bus system is called Controller Area Network (CAN) and is currently used not only in automobiles but also in industrial automation, for example in textile machines, packing machines, machines for manufacturing and distributing paper, and in medical technology. The serial bus is composed of a two-wire line, each of whose ends is provided with a bus terminal resistor.
CAN is a serial bus system that is multi-master capable, i.e., a plurality of CAN nodes can simultaneously request the bus. In CAN data transmission, according to the publication “Controller Area Network—a Serial Bus System Not Only for Motor Vehicles” of the International Association “CAN in Automation (CiA) e.V.,” no stations on the bus are addressed, but rather the content of the message is designated by a network-wide unambiguous identifier. In addition to recognizing the contents, the identifier also establishes the priorities of the message. The priorities are issued in the system design through corresponding binary values and are not dynamically changeable. The identifier having the lowest binary number has the highest priority. Conflict in bus access is resolved using bit-by-bit arbitration regarding the respective identifiers, in that each station, bit for bit, observes the bus level. In this competition among stations, all of the “losers” automatically become receivers of the message having the highest priority and only make the attempt once again to transmit when the bus becomes free. Upon the acceptance check occurring all receiver stations in the CAN network, after correctly receiving the message based on the identifier, determine whether the data received are relevant for it or not (selecting). If the data have meaning for the receiver stations, then they are further processed (acceptance), but otherwise they are simply ignored. The length of the information to be transmitted is relatively short. Eight bytes of useful data can be transmitted per message. Longer data blocks can be transmitted through segmenting. The maximum transmission speed is 1 MBit/s. This value applies to a bus system having an extension of up to 40 m. For distances up to 500 m, a transmission speed of 125 kBit/s is possible, and at transmission lengths of up to 1 km, a value of only 50 kBit/s is permissible. The number of users in one CAN bus system is theoretically limited by the number of available identifiers (2032 in standard format and 0.5·109 in expanded format). Therefore, CAN permits the realization of need-dependent bus access, proceeding, on the basis of the bit-by-bit arbitration, in a non-destructive manner through message priority. A synchronization mechanism is not supported by the CAN and the data transmission speed is too low for a process in which a plurality of sequences of motions proceed synchronously, one after the other.
One digital serial field bus system that supports a synchronization mechanism is the SERCOS interface (Serial Real Time Communication System). This SERCOS interface is a digital, serial communications system between control units and drive systems or input/output modules, and it is presented in greater detail in the article, “Communication in Drive Systems,” by Berthold Gick, Peter Mutschler, and Stephan Schultze, published in the German publication “etz”, Vol. 112 (1991), Issue 17, pp. 906–916.
A SERCOS interface specifies a rigidly hierarchical communication having data in the form of data blocks, the so-called messages, which are exchanged in temporally constant cycles between a control unit and a plurality of substations. Direct communication between the substations does not take place. The SERCOS interface makes use of the ring topology, there being present as users for each ring a control unit, also termed master, and a plurality of substations, also termed slaves. The physical layer of a transmission link is composed of optical point-to-point connections. The optical transmission takes place in a directed manner, the elements of the transmission link being electro-optical converters, fiber optics, and opto-electrical converters. The transmission rate is 2 MBit/s, 4 MBit/s, or 8 MBit/s. The length of each transmission segment in plastic optical fibers can be as much as 60 m, and in the glass optical fiber up to 250 m. The maximum number of users for each fiber-optic ring is 254. In addition, repeat amplifiers are arranged in the slaves so that signal distortions arising as a result of the optical transmission cannot accumulate. The active signal conditioning and clock-pulse regeneration is achieved with the assistance of phase-locking loops. By using fillers and bit stuffing, it is assured that a sufficient quantity of signal edges is contained in the data stream. As a result, it is made possible for the phase-locking loops always to remain “locked in place,” i.e., bit-synchronous.
Communication in the SERCOS interface is cyclical in operation, in the form of a master-slave communication having a cycle time that is selected during initialization. The master, in an independent transmitter signal element timing, either transmits messages or supplies filler to the ring. The slaves relay either their re-generated input signals to the next users (repeater function), or they transmit their own message. The master does not relay its input signal. For this reason, direct lateral (internode) communication between the individual slaves is not possible, and the ring can therefore be viewed as open at the master.
Each message begins and ends with a message boundary and has an address field, a data field, and a check sum. Messages that are transmitted by the slaves are source-addressed, i.e., the content of the address field indicates the transmitting station. Messages that the master transmits are destination-addressed. In the data field are the data to be transmitted. The length of the individual data fields of different messages is fixed during initialization and is then maintained at a constant value.
The communications cycle of the SERCOS interface is subdivided into five phases. The cycle begins with a master synchronization message, which functions to stipulate (input) the communications phase and the time reference. This is followed by the drive messages (source-addressed), which are transmitted by the individual slaves. After all the drive messages are present at the master, it transmits to all slaves a master data message. In addition, each slave, from the initialization time points, knows T3 and T4 within one cycle. At time point T3, system-wide, all data (setpoint values) are simultaneously released, and at time point T4, system-wide, all measuring values are simultaneously scanned. Immediately after the conclusion of the cycle time, the master begins the next cycle with the master synchronization message. Therefore, a SERCOS interface has the following synchronization types: bit synchronicity, synchronization of the communication, and the synchronization of data processing in the slaves.
Using this SERCOS interface, no rapid, direct-access communication that is also simple can be carried out among equal-access stations.
In the article, “How ‘Peer-To-Peer’ Aids Drive Technology,” published in the German publication “Engineering and Automation,” Vol. 16 (1994), Nos. 3–4, p. 48, a possibility is presented in which, within one multi-motor interconnection, signals are transmitted from drive unit to drive unit. Peer- to-peer connection denotes “connection between equal-access partners.” In this peer-to-peer connection, one and the same drive unit can be both master (setpoint value source) as well as slave (setpoint value acceptor). The peer-to-peer connection for each drive unit is composed of a receiving and a transmitting connection and of a two-wire line. On the basis of a peer-to-peer connection, an autonomous setpoint value cascade can be constructed, which is simple to configure and to place in operation. The transmission rate is up to 187.5 kBit/s, and it can be increased to up to 16 control signals. In this peer-to-peer connection, the expense for constructing a new communications sequence is quite high, since new two- wire lines must be laid.